I'm pretty sure "fear" of military invasion and economic depression would be a means for a regional military alliance between states and a likely independent economic cooperation zone between them, but no means to inspire everyone's foreign policy to be orchestrated by a single body. That would be like having to talk to Brussels to enter talks with Germany, but only deal with the Belgians. And that sort of control would probably sooner or later cause anger from within the member nations when the "senate body" makes unpopular foreign policy decisions on the guise that it's good for everyone. It's pretty much what's screwing up the European Union already, a international body seen by euro-skeptics as being irrelevant and detrimental to unique national issues is a terrible thing.

And the Hungarians and Romanians aren't Slavs.

The Hungarians are more culturally related to irrelevant Finns as being members of the Ugric family tree. 

The Romanians are their own very unique thing: Vlachs, Latinized Eastern Europeans living in parts of Northern Greece, Romania, and Moldovia. They speak a Latinized language, a Romance language like the French, Spanish, and Italians.

And issue with communication wouldn't be simply state-level, diplomats and statesmen would presumably already know a general language for diplomatic purposes (as Latin was classically used, then French, then English). The issue with the language barrier is between common people and to be able to read and speak it and use it intelligently.

Even between users of the same language bi-party trust may not always be there to properly run this sort of thing without it eventually being killed democratically or in violen revolution. As an example: though the US views Canada and Canadians highly, the Canadians don't view us in the same light. I believe the current positive opinion of the US in Canada is 40-50%.

And 10/10 replacing Romanians with gypsies. I'm sure there's a big diaspora loving Poland for promoting the inevitable gypsy-ran holocaust of a proud European native. It's like everything Eastern Europe feared. It's what the Nazis warned them about: those damn Jews and Gypsies would kill them all, pollute their proud, pure heritage!